这题不难，就是要理解清楚，然后注意一下各种细节就好。

class LRUCache {
    class Node{
        int key;
        int val;
        Node next;
        Node prev;
        public Node(int key, int val){
            this.key = key;
            this.val = val;
        }
    }
    int capacity;
    Node head;
    Node tail;
    Map<Integer, Node> map;
    public LRUCache(int capacity) {
        this.capacity = capacity;
        this.head = null;
        this.tail = null;
        this.map = new HashMap<>(capacity);
    }

    public int get(int key) {
        Node node = map.get(key);
        if (node == null){
            return -1;
        }
        if (node != tail){
            if (node == head){
                head = head.next;
            } else {
                node.prev.next = node.next;
                node.next.prev = node.prev;
            }
            tail.next = node;
            node.prev = tail;
            node.next = null;
            tail = node;
        }
        return node.val;
    }
    
    public void put(int key, int value) {
        Node node = map.get(key);
        if (node != null){
            node.val = value;
            if (node != tail){
                if (node == head){
                    head = head.next;
                } else {
                    node.prev.next = node.next;
                    node.next.prev = node.prev;
                }
                tail.next = node;
                node.prev = tail;
                node.next = null;
                tail = node;
            }
        } else {
            Node newNode = new Node(key, value);
            if (capacity == 0){
                int temp = head.key;
                head = head.next;
                map.remove(temp);
                capacity++;
            }
            if (head == null){
                head = newNode;
                tail = newNode;
            } else {
                tail.next = newNode;
                newNode.prev = tail;
                newNode.next = null;
            }
            tail = newNode;
            map.put(key, newNode);
            capacity--;
        }
    }
}

/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache obj = new LRUCache(capacity);
 * int param_1 = obj.get(key);
 * obj.put(key,value);
 */